Understanding the fundamental molecular mechanisms underlying megakaryocytic differentiation and polyploidization is an essential first step to discovering novel therapeutic targets and approaches for treating Mk and platelet disorders such as essential thrombocytopenia, refractory thrombocytopenia in myelodysplastic syndromes, megakaryoblastic leukemia, and thrombocythemia. Myelodysplastic syndromes (MDS) are heterogeneous hematopoietic stem cell disorders often accompanied by defective megakaryocyte (Mk) development, decreased platelet counts, and often progress into leukemia. Most MDS patients have chronic thrombocytopenia, but platelet transfusion is complicated by the development of allogeneic antibodies and ca. 30% of MDS patients die of bleeding. Therefore, increased endogenous platelet production could extend the lifespan (and increase the quality of life) for a substantial fraction of MDS patients by rescue from and/or delayed onset of clinically significant thrombocytopenia. Unfortunately, little information is known about Mk (platelet precursors) or platelet production. Because the number of platelets released by a single Mk is directly proportional to how much DNA the cell contains, understanding the Mk cell cycle may provide crucial insights and allow for better platelet expansion. Well-known as a "toxic sensor," aryl hydrocarbon receptor (AhR) function is normally associated with the action mechanism of various environmental toxins, presumably by altering cell cycle regulation. Environmental exposure to high levels of AhR ligands has been associated with increased platelet counts and we hypothesize that as a cell cycle regulator, AhR may impact Mk polyploidization, a process known to involve a modified cell cycle. Our data indicate that AhR expression and activity increases during Mk differentiation of both cell lines and primary human CD34+ cells and coincides with increased target gene expression. Importantly, AhR RNAi knockdown results in decreased Mk polyploidization through unknown mechanisms. We propose to (1) test the hypothesis that AhR is required for the formation ofproplatelet extensions and in vivo platelet production and function, (2) further investigate if AhR is activated and necessary for megakaryopoiesis, leading to increased Mk polyploidization;(3) determine if AhR-mediated Hes1 expression regulates the level and localization of cyclin D3, cyclin E, and other key Mk genes during megakaryopoiesis;Our experiments could simultaneously provide important insights into megakaryopoiesis by identifying a novel thrombocytopenic event (reduced AhR), and guide new treatment options to increase platelet counts (drugs and/or treatments that increase AhR activity) in thrombocytopenic patients.